1. Field of the Invention
This invention relates to an improved method for bleaching alkylpolyglycosides, and in particular to a continuous method of bleaching with a peroxide, such as hydrogen peroxide.
2. Description of Related Art
Alkyl glycosides are conveniently prepared by reacting an alcohol of the type and chain length which is desired to form the "alkyl" portion of the glycoside of interest with a saccharide reactant (e.g., a monosaccharide such as glucose, xylose, arabinose, galactose, fructose, etc., or a polysaccharide such as starch, hemicellulose, lactose, maltose, melibiose, etc.) or with a glycoside starting material wherein the aglycone portion thereof is different from the alkyl substituent desired for the ultimate alkyl glycoside product of interest. Typically, such reaction is conducted at an elevated temperature and in the presence of an acid catalyst. Various alkyl glycoside products and processes for making same are disclosed in a variety of representative patents. U.S. Pat. No. 4,987,225 contains an extensive listing of processes for preparing alkyl glycoside compositions. As disclosed therein, processes for preparing alkyl glycoside compositions are disclosed in U.S. Pat. No. 3,219,656 to Boettner (issued Nov. 23, 1965); U.S. Pat. No. 3,547,828 to Mansfield et al., (issued Dec. 15, 1970); U.S. Pat. No. 3,598,865 to Lew (issued Aug. 10, 1971); U.S. Pat. No. 3,707,535 to Lew (issued Dec. 26, 1972); U.S. Pat. No. 3,772,269 to Lew (issued Nov. 13 1973); U.S. Pat. No. 3,839,318 to Mansfield (issued Oct. 1, 1974); U.S. Pat. No. 4,349,669 to Klahr (issued Sep. 14, 1982); U.S. Pat. No. 4,393,203 to Mao et al. (issued Jul. 12, 1983); U.S. Pat. No. 4,472,170 to Hellyer (issued Sep. 18, 1984); U.S. Pat. No. 4,510,306 to Langdon (issued Apr. 9, 1985); U.S. Pat. No. 4,597,770 to Forand et al. (issued Jul. 1, 1986); U.S. Pat. No. 4,704,453 to Lorenz et al. (issued Nov. 3, 1987); U.S. Pat. No. 4,713,447 to Letton (issued Dec. 15, 1987); published European Application No. 83302002.7 (EPO Publication No. 0092355; Vander Burgh et al; published Oct. 26, 1983); published European Application No. 83200771.0 (EPO Publication No. 0096917; Farris; published Dec. 28, 1983); and published European Application No. 84303874.6 (EPO Publication 0132043; published Jan. 23, 1985). Other representative patents are U.S. Pat. No. 2,235,783 (White, issued Mar. 18, 1941); U.S. Pat. No. 2,356,565 (Chwala, issued Aug. 22, 1944); U.S. Pat. No. 2,390,507 (Cantor, issued Dec. 11, 1945); U.S. Pat. No. 2,422,328 (Young, issued Jun. 17, 1947); U.S. Pat. No. 3,375,243 (Nevin et al., issued Mar. 26, 1968); U.S. Pat. No. 3,450,690 (Gibbons et al., issued Jun. 17, 1969); U.S. Pat. No. 3,640,998 (Mansfield et al., issued Feb. 8, 1972); U.S. Pat. No. 3,721,633 (Ranauto, issued Mar. 20, 1973); U.S. Pat. No. 3,737,426 (Throckmorton et al., issued Jun. 5, 1973); U.S. Pat. No. 3,974,138 (Lew, issued Aug. 10, 1976); U.S. Pat. No. 4,011,389 (Langdon, issued Mar. 8, 1977); and U.S. Pat. No. 4,223,129 (Roth et al., issued Sep. 16, 1980).
In the preparation of alkyl glycoside products, it is not uncommon for such products to develop an undesirably dark coloration during the course of the synthesis and isolation procedures employed. Various procedures have been suggested for improving the color of such dark colored glycoside products including, for example, treatment with bleaching reagents such as hydrogen peroxide; intentional color formation by heat treatment under alkaline conditions followed by removal (e.g., by precipitation, filtration, etc.) of dark colored impurities generated during said treatment procedure; treatment with decolorizing adsorbents such as particulate carbon materials, etc.; and the like. See in this regard, for example, Gibbons U.S. Pat. No. 3,450,690 which discloses an alkaline heat treatment/separation procedure that can optionally be followed by treatment with bleaching agents such as hydrogen peroxide or by treatment with decolorizing carbons. See also Cantor's U.S. Pat. No. 2,390,507; White's U.S. Pat. No. 2,235,783; Example 1 of Throckmorton et al.'s U.S. Pat. No. 3,737,426; Examples 5 and 10 of Langdon's U.S. Pat. No. 4,011,389; and Example 1 of U.S. Pat. No. 4,472,170 to Hellyer (issued Sep. 18, 1984) for teachings related to the use of carbon adsorbents for the decolorization of various alkyl glycoside products.
Even when glycoside products are originally prepared (or are subsequently decolorized in accordance with one or more of the procedures set forth above) in a fashion which results in initial color characteristics acceptable for certain applications, such products nonetheless commonly exhibit a propensity to discolor (i.e., darken) as a function of time even under relatively mild storage conditions (e.g., at neutral or slightly acidic pH and ambient conditions, i.e., 20.degree. C.-35.degree. C.). The propensity to discolor is greatly accentuated (i.e., in terms of the intensity and rapidity thereof) by exposure to elevated temperatures (such as, for example, in the range of 35.degree. C. to 100.degree. C. or more) and/or exposure to relatively strong alkaline aqueous environments (i.e., pH of 8 to 12). Generally speaking, the extent of discoloration is related to the severity of the pH/temperature/time to which the glycoside product is exposed. In U.S. Pat. No. 4,557,729 to McDaniel et al. (issued Dec. 10, 1985), the aforementioned problem of color deterioration of glycoside products during storage thereof is discussed and a method for obviating such problem is disclosed which entails first bleaching the glycoside product of interest with an oxidizing agent such as ozone, hydrogen peroxide, hypochlorite, etc., and thereafter exposing the resulting bleaching glycoside product to a source of sulfur dioxide (e.g., sulfur dioxide gas, sodium sulfite, sodium metabisulfite, sodium hydrosulfite, etc.) to stabilize said glycoside product against color degradation. Another McDaniel et al. patent, U.S. Pat. No. 4,904,774, notes the discoloration tendency of glycosides which have been decolorized by bleaching with peroxide materials such as hydrogen peroxide, upon exposure to high temperatures, and proposes color reduction by hydrogenation under catalytic hydrogenation conditions using materials such as Raney nickel or sodium borohydride. U.S. Pat. No. 4,990,605 to Lueders (issued Feb. 5, 1991) describes a method of manufacturing light colored alkyloligolglycosides by treatment with activated carbon followed by distillation and bleaching with a peroxide compound, preferably hydrogen peroxide, at temperatures of 50.degree. to 100.degree. C. under neutral or alkaline pH. Example 1, and comparative Example A, illustrates and compares the process with and without the activated carbon treatment.
The overall process to prepare light colored alkylpolyglycoside surfactants accordingly typically involves reaction of an alcohol with a saccharide in the presence of an acid catalyst followed by neutralization of the acid catalyst, removal of the alcohol and bleaching of the resultant substantially alcohol-free alkylpolyglycoside product, followed usually by a stabilization treatment to provide color stability. In the past, the process has been conducted as a batch process, and while it was recognized that a continuous process would be desirable, no practical continuous process has been developed to provide a very light colored alkylpolyglycoside surfactant product, other than the process described in commonly assigned, copending application U.S. Ser. No. 07/914,363 filed Jul. 15, 1992. In that application there is described a continuous process for reducing the color of an alkylpolyglycoside by bleaching, preferably with hydrogen peroxide and caustic in the presence of Mg. While this process avoided the disadvantages of previous approaches in batch processes, even in such process, however, difficulties may be encountered particularly in large scale, commercial production runs. Typically it takes a certain minimum time to get the reactor bleaching zone "lined out", to reach steady state conditions resulting in increased chemical usage and poorer color during startup until steady state continuous operation is achieved. A detailed description of that continuous process, on which the present invention is an improvement, will be discussed in more detail hereinafter.